Hydraulic excavators, for example, are conventionally used for a wide variety of operations such as excavation, craning and leveling and therefore are required to have the capability of performing effective excavation, while keeping fine controllability needed for craning and leveling operations.
One hydraulic excavator which meets the above requirement is set out in, e.g., Japanese Patent Publication No. 3316057. The hydraulic excavator proposed in this publication includes hydraulic actuators activated by pressure oil from a variable displacement hydraulic pump driven by an engine; an operating speed detecting means for detecting the change rate of operation amount of a control lever for operating a hydraulic actuator; and an engine revolution speed controlling means for controlling engine revolution speed. If the change rate of operating amount detected by the operating speed detecting means is lower than a specified value, the revolution speed of the engine is kept to a preset speed by the engine revolution speed controlling means. Specifically, during the operation in which the change rate of operating amount of the control lever is small such as craning and leveling, increasing/decreasing of engine revolution speed is inhibited so as not to affect the fine control. On the other hand, during the operation in which the change rate of operating amount detected by the operating speed detecting means is higher than the specified value, the engine revolution speed controlling means increases the revolution speed of the engine from the preset speed according to the load imposed on the hydraulic actuator. That is, since the change rate of operating amount of the control lever during excavation is higher than those of craning and leveling operations, the revolution speed of the engine is increased according to work load, thereby effectively performing excavation.
The hydraulic excavator disclosed in the above publication, however, cannot avoid complication of the control system, because it is designed to control the revolution speed of the engine by the engine revolution speed controlling means based on the change rate of operating amount of the control lever detected by the operating speed detecting means. In addition, since the threshold for determining whether the change rate of operating amount is high or low is set based on the operational feeling of the operator, the recall factor of the operational effect is likely to vary under the influence of the physical condition of the operator or the individual difference between the operators when a plurality of operators use the work machine in cooperation.
The present invention is directed to overcoming the foregoing problem and a primary object of the invention is therefore to provide a work machine with an engine control device which is capable of ensuring reliable fine controllability and providing further improved fine controllability with a relatively simple construction.